It's pretty darn easy with the Digi Sync. You may see also be able to detect this with gauges or a fluid manometer but I haven't tried. I recently purchased a couple of carburetor diaphragms (thanks Bert) with the intention of damaging them. It wasn't too hard as one of them was already torn in a couple of places and that's just what I wanted!
First, I synched my Interstate at 900 rpm to within +/-1 counts on every cylinder. I then revved the engine throughout a range up to 3K rpm. I dwelled at several spots to let the values settle. The Digi Sync counts stayed very close to one another within a couple of counts.
Then I exchanged one of my good diaphragms with a bad one.
At 900 rpm the sync was still good (You're right Ricky-D) but by revving the engine anywhere from 1K to 2K rpm it was easy to see that I had a bad diaphragm and, of course, which cylinder it is! The Digi Sync counts started to deviate noticeably at 1K and continued to show more error as the rpm's climbed. At 2K the bad carb/cylinder was about 10-15 counts
down from the others and fluctuating.
I suppose that the amount of leakage through the tear will determine the amount of sync error but good carbs/diaphragms shouldn't deviate from one another by more than a couple of counts above idle by this experience.
I'm going to put this newly learned wisdom into the Digi Sync instructions as it's a really easy test to do and points you right to the problem carburetor like x-ray vision.
